There is some difference of opinion regarding the use of apostrophes in the pluralization of references to letters as symbols. New Fowler's Modern English Usage, after noting that the usage has changed, states on page 602 that "after letters an apostrophe is obligatory." The 15th edition of The Chicago Manual of Style states in paragraph 7.16, "To avoid confusion, lowercase letters ... form the plural with an apostrophe and an s". The Oxford Style Manual on page 116 advocates the use of common sense.